Such a process has been disclosed in U.S. Pat. No. 4,105,469. In that process, the spent pickling solution used to pickle zirconium or a zirconium alloy is to be regenerated to a Zr content of 3 to 7 g/l by an addition of NaF in an adequate amount. It is desired to precipitate Na.sub.2 ZrF.sub.6 because NaZrF.sub.5 is allegedly precipitated as a gel and can be filtered only with difficulty.
In U.S. Pat. No. 4,330,342, it has been stated that the process known from the U.S. Pat. No. 4,105,469 is rather expensive and can be carried out only with difficulty. It has been stated there that the relatively expensive precipitant must be added in crystalline form to ensure that the pickling solution will not be too strongly diluted and that the process is highly susceptible to an addition of the precipitant in an inadequate or excessive amount. The addition of an expensive amount might result in an excessive residual NaF content so that Na.sub.2 ZrF.sub.6 might already be precipitated in the pickling bath whereas in case of an addition of an inadequate amount, a gel-like NaZrF.sub.5 .times.H.sub.2 O would be formed and could be filtered-off only with difficulty. For this reason, it has been proposed to heat the spent pickling solution above 40.degree. C., to add dissolved NaOH as a precipitant, to cool below 20.degree. C. and then to separate by filtration the precipitated Na.sub.2 ZrF.sub.6. NaOH should be added in such an amount that the fully regenerated pickling solution has a residual Na content of 1 to 3 g/l, preferably 1.5 to 2.5 g/l, and a residual Zr content of 1.5 to 2.5 g/l.
However it has been found that considerable difficulties are involved in the practice of the aforedescribed process. These difficulties are mainly due to the fact that the regenerated pickling solution strongly tends to become supersaturated, which particularly in continuous operation, results in a formation of hard salt deposits throughout the system. Even after a short operating time, the process has to be interrupted because flow paths are clogged by deposits and must be cleaned. An economical regeneration of the pickling solution cannot be achieved.